1. Field of the Invention
This invention relates to delta modulation and, more particularly, to delta modulation which partitions the input signal into variable-time segments that are iteratively encoded.
2. Description of the Prior Art
The arts of delta modulation and of speech encoding through delta modulation are well established, and many articles teach the uses and advantages of these arts. A good exposition of delta modulation and of its particular use in speech encoding may be found in "Delta Modulation," by H. R. Schindler, IEEE SPECTRUM, October 1970, pp. 69-78. Delta modulation may be succinctly described as the encoding of an analog signal into digital format wherein the analog signal is converted to a sequence of binary pulses having values that depend on the relationship between the amplitude of the analog signal at the instant of occurrence of each pulse and the previous amplitude of the analog signal. More specifically, in a delta modulation encoder, the input signal is synchronously tested in a comparator against an estimate of the signal as developed from an integral of the comparator's output signal. Whenever the estimate of the signal is too low, the comparator's output is set high, and whenever the estimate is too high, the comparator's output is set low.
As in many other modulation schemes, various trade-offs must be made when encoding in delta modulation. For example, a higher clock frequency provides a better approximation to the input signal but requires a higher transmission bandwidth, a larger step size affords a larger dynamic range and faster response but increases quantization noise, and so forth.
To combat some of the disadvantages resulting from these trade-offs, the concept of adaptive delta modulation (ADM) has been invented which, generally, controls the step size of the delta modulator in response to the input signal's past characteristics. The concept of adaptive delta modulation is described, inter alia, in "Linear and Adaptive Modulation," J. E. Abate, Proceedings of the IEEE, Volume 55, No. 3, March 1967, pp. 298-307; and in "Adaptive Delta Modulation With One-Bit Memory," N. S. Jayant, Bell System Technical Journal, Volume 49, No. 3, March 1970, pp, 321-342. Also, U.S. Pat. Nos. 3,652,957, issued to Goodman on Mar. 28, 1972, 3,784,922, issued to Blahut on Jan. 8, 1974, and 3,806,806, issued to S. J. Brolin on Apr. 23, 1974, describe novel embodiments of adaptive delta modulators.
Although it is generally true that the step size in an adaptive delta modulator is controlled by the input signal's past characteristics, the exact relationship between the step size and the input signal's past variations differs with different implementations of ADMs and, consequently, the system operations of different ADMs also differ.
For the illustrative purposes of this disclosure, the ADM described by Brolin in the aforementioned U.S. patent is employed, and therefore, its operation is briefly described below. It should be noted, however, that the method of this invention is useful for all delta modulators.
In the Brolin ADM, the estimate of the input signal (estimate signal) is maintained across a "leaky" capacitor which causes the estimate voltage to continually decay. At each clock pulse of the ADM, a current pulse is either added, when the estimate voltage is too low, or extracted, when the estimate voltage is too high. The magnitude of that current pulse is related, though nonlinearly, to the number of consecutive "0" or "1" logic states in the encoder's past outputs, and the estimate voltage (i.e., the capacitor voltage) follows the equation EQU Ve.sup.-k.sbsp.1.sup.t .+-.I/(Ck.sub.2) (1-e.sup.-tk.sbsp.2) (1)
where V is the voltage across the capactior at the instant the current pulse is applied, k.sub.1 is the passive decay time constant of the capacitor, I is the amplitude of the current pulse, K.sub.2 is the active charge or discharge time constant of the capacitor due to the current pulse and C is the capacitor value. Generally, the constant k.sub.2 is much larger than the constant k.sub.1.
The response of this ADM can be more fully understood from the discussion below, read in conjunction with the FIG. 1 drawing which depicts an input signal 10 and its corresponding estimate voltage 20.
When the input signal is close to zero, as at point 11 in FIG. 1, the encoder's output generally alternates between the "1" and "0" states, the amplitude of the current pulse is small and the estimate voltage is alternating between a positive and a negative potential, as at points 21 and 22. Pictorially, the alternating estimate voltage resembles the triangular voltage of a capacitor driven through a resistor by a square wave source.
When the input voltage suddenly increases in magnitude, the number of consecutive "1" or "0" states increases, the magnitude of the current pulse increases and the estimate (capacitor voltage) starts to follow the input voltage. As the estimate voltage increases, the first term in equation 1 assumes a progressively larger control and attempts to reduce the estimate voltage, thus bucking the current pulse drive of the second term in equation 1. In consequence of this bucking action, the increases in the estimate voltage from one clock pulse to the next are smaller as the estimate voltage increases (in the positive or negative direction). This is depicted by the voltage differences between points 23 and 24, and 25 and 26.
This reduction of the increases in the estimate voltage is generally desirable, because as the input voltage increases, the chances of its slope reducing are higher, and the reduced increases in the estimate voltage anticipate exactly that occurrence.
When the input signal does peak and reverses direction, the passive decay of the capacitor (the first term in equation 1) now adds to the reversed direction of the voltage steps induced by the current pulse. At first, the current pulse amplitude is small because of the transition in the modulator's output bits. Later, the current pulse amplitude is increased again, resulting in fairly large capacitor voltage variations until some time after the estimate voltage crosses the zero axis; at which time the passive decay again assumes a bucking function.
Even with adaptive delta modulation, the step size of the modulator is directly dependent only on the input signal's history (usually, a finite history), and, accordingly, the estimates of the input signal are not as good as they could be. This loss, which occurs in both linear and adaptive delta modulation processes, is observable in FIG. 1, by the constant lag of signal 20 with respect to input signal 10.
In an effort to reduce the delta modulation error resulting from the nonuse of the signal's future behavior, a tree encoding technique has been proposed by J. B. Andersen in "Tree Encoding of Speech," IEEE Transactions on Information Theory, Volume IT-21, July 1975, pp. 379-387. Briefly, tree encoding involves an exhaustive search for a best sequence of concatenated segments. Expectedly, the number of computations involved in a tree encoding search expands geometrically as the number of bits encompassed by the signal's segment is increased. Each additional bit in the search tree doubles the computation effort. For most applications, this approach is too costly.